The gods of evolution had served a long apprenticeship; they had gained proficiency and were master workmen.  Or shall we say that the elements of life had become more plastic and adaptable, or that the life fund had accumulated, so to speak?  Had the vast succession of living beings, the long experience in organization, at last made the problem of the origin of man easier to solve?

One fancies every living thing as not only returning its mineral elements to the soil, but as in some subtle way leaving its vital forces also, and thus contributing to the impalpable, invisible store-house of vital energy of the globe.

At first among the mammalian tribes there was much muscle and little brains.  But in the middle Tertiary the mammal brain began suddenly to enlarge, so that in our time the brain of the horse is more than eight times the size of the brain of his progenitor, the dinoceras of Eocene times.

Nature seems to have experimented with brains and nerve ganglia, as she has with so many other things.  The huge reptilian creatures of Mesozoic time—­the various dinosaurs—­had ridiculously small heads and brains, but they had what might be called supplementary brains well toward the other end of the body,—­great nervous masses near the sacrum, many times the size of the ostensible brain, which no doubt performed certain brain functions.  But the principle of centralization was at work, and when in later time we reach the higher mammalian forms, we find these outlying nervous masses called in, so to speak, and concentrated in the head.

Nature has tried the big, the gigantic, over and over, and then abandoned it.  In Carboniferous times there was a gigantic dragon-fly, measuring more than two feet in the expanse of wings.  Still earlier, there were gigantic mollusks and sea scorpions, a cephalopod larger than a man; then gigantic fishes and amphibians and reptiles, followed by enormous mammals.  But the geologic record shows that these huge forms did not continue.  The mollusks that last unchanged through millions of years are the clam and the oyster of our day.  The huge mosses and tree-ferns are gone, and only their humbler types remain.  Among men giants are short-lived.

On the other hand, the steady increase in size of certain other species of animals during the later geologic ages is a curious and interesting fact.  The first progenitors of the elephant that have been found show a small animal that steadily grew through the ages till the animal as we now find it is reached.  Among the invertebrates this same progressive increase in size has been noted, a small shell in the Devonian becoming enormous in the Triassic.  Certain species of sharks of medium size in the lower Eocene continue to increase till they attain the astounding dimensions in the Miocene and Pliocene of over one hundred feet long.  A certain fish appearing in the Devonian as a small fish of seven centimetres in length, becomes